At the present time, solely zinc or zinc alloy coatings provide reinforced protection against corrosion via twofold protection of barrier and of cathodic type. The barrier effect is obtained by applying the coating to the steel surface which prevents any contact between the steel and the corrosive medium and is independent of the type of coating and substrate. On the contrary, sacrificial cathodic protection is based on the fact that zinc is a less noble metal than steel and that, under corrosion conditions, it is consumed in preference to steel. This cathodic protection is essential in particular in areas where the steel is directly exposed to a corrosive atmosphere such as cut edges or damaged areas where the steel is exposed and the surrounding zinc will be consumed before any attack of the non-coated area.
However, because of its low melting point zinc gives rise to problems when parts need to be welded, since there is a risk that it may vaporize. To overcome this problem, one possibility is to reduce the thickness of the coating but in this case the lifetime of corrosion protection is limited. In addition, if it is desired to press harden a sheet, in particular by hot drawing, micro-cracks are seen to form in the steel which propagate from the coating. Also, the painting of some parts previously coated with zinc and press hardened require a sanding operation before phosphatation because of the presence of a fragile oxide layer on the surface of the part.
The other family of metal coatings frequently used to protect automobile parts is the family of coatings based on aluminum and silicon. These coatings do not generate any microcracking in steel during the forming process because of the presence of an intermetallic Al—Si—Fe layer, and they lend themselves well to paint application. While they allow protection to be obtained via a barrier effect and can be welded, they do not however allow any cathodic protection to be obtained.
Application EP 1 997 927 describes corrosion-resistant steel sheet coated with a coating comprising more than 35% by weight of Zn and comprising a phase in non-equilibrium having a heat capacity measured by differential scanning calorimetry of 1 J/g or higher, typically having an amorphous structure. Preferably, the coating comprises at least 40% by weight of zinc, 1 to 60% by weight of magnesium and 0.07 to 59% by weight of aluminum. The coating may comprise 0.1 to 10% lanthanum to improve the ductility and workability of the coating.
It is one of the objectives of the present application to overcome the disadvantages of prior art coatings by providing coated steel sheets having reinforced protection against corrosion, in particular before and after production by drawing. If the sheets are intended to be press hardened, in particular hot drawn, resistance against the propagation of microcracking in the steel is also sought and preferably with an operating window that is as wide as possible regarding time and temperature during heat treatment prior to press hardening.
In terms of sacrificial cathodic protection, it is sought to reach an electrochemical potential at least 50 mV more negative than that of the steel i.e. a minimum value of −0.78V relative to a saturated calomel electrode (SCE). However, it is not desired to go below a value of −1.4V, even −1.25V as this would cause too rapid consumption of the coating and reduce the lifetime of steel protection.